This invention relates generally to carburetor devices and, more particularly, to gas turbine engine combustion systems having low pressure central fuel injectors.
Engine manufacturers are constantly striving for a carburetor design which will provide a fuel/air mixture to a continuous-flow combustion chamber for the achievement of complete and efficient combustion of the fuel by minimizing the occurrence of fuel-rich pockets which, upon combustion, may produce carbon or smoke. The attainment of complete combustion is complicated by the ever-present desire to increase engine efficiency and, therefore, combustor pressure inlet temperature and exit temperature. Existing fuel spray atomizer performance deteriorates as combustor pressure is increased due to the requirement for operating over a broad range of conditions. This results in a more nonuniform dispersion of fuel which may cause a nonuniform heating of the combustor shell and hot streaks in the turbine, as well as potential carbon and smoke-producing problems.
The conventional spray atomizers with their high pressure fuel systems are being replaced by low pressure fuel systems having counterrotational primary and secondary swirl vanes which efficiently atomize the fuel by the high shear forces developed at the confluence of the counterrotational airstreams. The most common counterrotational system employs, in the primary stage, an axial swirler where the air enters in an axial direction, is deflected in a somewhat circumferential direction to introduce a swirl to the airflow, and then flows axially downstream within the venturi where it finally mixes and interacts with the air from the counterrotational secondary swirler. One disadvantage of such a system is that, due to the relatively low velocity air introduced at the root of the axial swirler, a deposit of carbon is likely to be formed on the fuel injector, which in turn may affect the flow of fuel and thereby the efficiency of the overall system. Another disadvantage to the axial primary swirler is that of its required axial length which necessitates the extension of, or cutouts on, the combustor cowl which are undesirable due to a lack of structural rigidity and the resultant nonuniform flow path. In addition, an increase in engine length may be required. Further, due to the difference in temperature and thermal response between the outer casing (which determines the location of the fuel injector) and the combustor dome, it is necessary to provide a slip joint to allow for relative thermal growth. In a carburetor system having an axial primary swirler, the location of such a slip joint is likely to cause eccentricity between the primary and secondary airflows to thereby disrupt the resulting swirl flow from the secondary swirler.
Other systems have suggested that a fuel/air mixture be introduced upstream of the swirl vanes, whereupon the fuel becomes subsequently atomized upon shearing of the liquid fuel droplets from the swirl vanes. Such atomizers have been found on occasion to accumulate carbon between the swirl vanes when the inlet airflow and fuel delivered to the atomizers are at relatively high temperature levels. Further, under some combustor operating conditions, fuel decomposition may occur and the resulting formation of carbon deposits within the premixing scroll may tend to restrict the entry of fuel/air mixture into the combustor dome and may possibly lead to fuel spillage out of the scroll inlet and into the air upstream of the dome.
Accordingly, a primary object of this invention is to provide an improved carbureting device for introducing a fuel/air mixture into a combustion chamber for efficient, low emission and low-smoke combustion of the fuel.
Another object of this invention is the provision for the delivery of fuel to the carburetor by a low pressure fuel system which does not allow the formation of carbon on the fuel injector.
Still another object of this invention is the provision for a carburetion device which is relatively short in length and therefore easy to install in a conventional combustor cowl.
Yet another object of this invention is the provision for accommodating the differential expansion and tolerances between elements within the combustor's dome without disrupting the uniform fuel distribution.
A further object of this invention is the provision for a carbureting device which does not allow the backup of fuel outside the combustor.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.